a. Field of the Invention
This invention relates to blade opening and closing mechanisms for electric shutters and more particularly to a mechanical exposure controlling means for electric shutters wherein, in case an electromagnet for controlling the shutter closing time does not work properly due to the drop of the voltage of the current source battery to be below a predetermined value or to a failure of the exposure controlling circuit, by a manual switching operation, an exposure will be made at a specific shutter speed determined in the design or a bulb exposure will be made.
B. Description of the Prior Art
In an electric shutter, in case an electromagnet for controlling the shutter closing time is not energized well or at all due to the drop of the voltage of the current source battery to be below a predetermined level or to a failure of the circuit, the exposure time to be controlled will naturally become improper but, depending on the kind of the shutter, in such case, the rear blade actuating member will start the motion prior to the front blade actuating member and no actual exposure will be made at all.
Therefore, there has been already proposed an electric shutter wherein, by taking the above described point into consideration, an exposing operation can be made at a specific shutter speed by a mechanical controlling means independently of the proper exposure. However, in this kind of conventional means, there have been such defects as are mentioned in the following. That is to say, a conventional product is an electric shutter comprising an armature lever to which a force separating from an electromagnet is given, a rear blade locking lever to be pushed and moved in the course of the separating motion of said lever and a rear blade actuating member to be engaged with said locking lever, wherein a mechanical exposure controlling member is brought into the motion range of the armature lever so that said armature lever may be once engaged with said mechanical exposure controlling member in the motion process before said armature lever pushes and moves said locking lever. Therefore, in some case, due to the fluctuation of the precision of parts, before being engaged with the mechanical exposure controlling member, the armature lever will push and move the rear blade locking lever to immediately operate the rear blade actuating member. Further, the separating force of the armature lever is set to be as weak as possible to reduce the consumption of the battery in the relation with the attraction of the electromagnet. Therefore, the separation force of the armature lever from the position in which the armature lever is once engaged with the mechanical exposure controlling member will become so weak that the armature lever will not be able to push and move the rear blade locking lever in the relation of the balance in the rear blade locking lever and the operation force of the rear blade actuating member and will be no longer able to release the rear blade actuating member in some case.